EXP works differently inside Abyssea than the rest of the game. There is no party size EXP penalty, so you get the same amount of EXP solo as you do in an Alliance of 18 people. The amount of EXP you get per kill also increases as you build a "Chain", which, as opposed to normal EXP chains outside Abyssea, simply builds as you kill the same type of monster repeatedly rather than killing in rapid succession. You generally peak at about 600 EXP/kill, with some mob types capping higher or lower. Mobs also drop chests that can be worth up to 1,250 EXP on their own, and there are "Dominion Ops" in Heroes of Abyssea zones which net you about 3-5k bonus EXP for every 5 mobs killed. Overall, parties can hit up to 200,000 EXP/hour if they're good.
The EXP level correction still exists. A level 30 in Abyssea still receives 33% EXP if they are in a party with a level 90, but because Abyssea EXP/kill rises so high, even a level 30 player can hit 600 EXP/kill after enough of a chain has been built.
EXP outside Abyssea, in the normal game areas, has also received a global Doubling. Also, Easy Prey and Decent Challenge monsters had their base EXP/kill increased. Overall, EXP has become a means to an end rather than content itself.
As for Endgame content, everything that was present at 75 still exists now for those who wish to pursue it. I am told by some posters on this forum that it is "impossible" to find random people who are willing to do old content, because the gear dropped in Abyssea so easily outclasses old armor that they feel there is little point to it. However, again, this is just another reason why I would recommend trying to find a core group of friends you can play with. I still do sea, Einherjar, Dynamis, and pretty much anything else that's actually moderately enjoyable with friends.
Abyssea itself is 9 zones worth of content, ranging from EXP, to timed NMs, to force pop HNMs. All Empyrean Weapons are constructed using drops from NMs in Abyssea, and the new "AF3" armor is upgraded there.
AF3 armor starts as an "NQ" piece with practically no stats, either bought with Cruor (the "Conquest Points" of Abyssea), or in some other fashion from an NPC. It is then upgraded into AF3+1, the "Casual" player's AF3, by obtaining a certain number of AF3 Seals. Seals are dropped by low-tier NMs, and are each designated with both a Job and a Slot (Eg: A monster might drop Bard Seal: Legs). Scars of Abyssea zones (Attohwa, Misareaux, and Vunkerl) drop Head seals, Leg seals, and Feet seals respectively. Heroes of Abyssea zones (Uleguerand, Altepa, and Grauberg) drop Body and Hand seals.
AF3 armor is then upgraded into AF3+2 by procuring "Jewels" (rather, Jewels, Stones, Cards, and/or Coins depending on the job). Like with seals, each "Jewel" (I'll use this term to mean all 4 objects) represents a single slot. However, unlike seals, each Jewel is used for multiple jobs AF3. Eg: Stone of Balance is used to upgrade WAR, BLM, BLU, PUP, and NIN AF3+1 feet into AF3+2 feet. AF3+2 also has a Set Bonus, which activates when wearing 2 or more pieces of the set, increasing in chance to trigger as you add more pieces.
Non-Body AF3 take 8 seals to upgrade from NQ to +1, and 6 "Jewels" to upgrade from +1 to +2. Body AF3 takes 10 seals to upgrade from NQ to +1 and 9 "Jewels" to upgrade from +1 to +2.
The current focus of the game is on building AF3+2 sets and Empyrean Weapons in order to prepare for level 95-99 content. It may get a tad repetitive, but it's extremely fun (in my opinion, since you can basically become a Demigod in Abyssea), and the content can be handled by just about anyone.
As for SMN and PLD, they're just really not that useful. The playerbase as a whole has realized that DD jobs just aren't as "frail" as people thought they were. More DDs carry PDT and MDT (Physical Damage Taken -% and Magical Damage Taken -%) sets with them, allowing them to mitigate damage if they need to. Combined with the fact that WHMs have practically infinite MP in Abyssea, and a good MNK is sitting on 3.6k-4k HP, there really isn't any need at all for a PLD. They're just lackluster DDs with overkill Defensive abilities that don't really serve much purpose (I say this as a long-time lover of the PLD job, too).
SMN has just always been iffy. The 45 second cap on BP Delay and the inability to transfer high Haste buffs to pets really knocks them down a few notches. A good SMN can still lop out a 5k Blood Pact every 45 seconds, but that doesn't mean too much when my Monk lops out a 5k WS every 10-15 plus TP phase damage (numbers are for inside Abyssea, aka Demigod mode)